The present invention relates to a driving apparatus of a linear compressor for reciprocating a piston in a cylinder by a linear motor to generate compressed gas in a compression chamber formed by the cylinder and the piston.
Conventionally, a linear compressor utilizing elasticity of a mechanical elastic member or compressed gas is known as means for generating compressed gas.
To efficiently drive the linear compressor, it is necessary to drive the linear compressor at resonance frequency of the linear compressor. In a linear compressor having the elastic member, the resonance frequency of the linear compressor is determined by the elastic member (mechanical spring) which is mechanically provided and elasticity (gas spring) generated by compressed gas. In a linear compressor utilizing only elasticity of compressed gas, the resonance frequency is determined only by the elasticity. However, since the elasticity generated by the compressed gas is largely varied with variation of load, the resonance frequency of the linear compressor can not be determined as one value. Therefore, the conventional technique employs a method for calculating the varying resonance frequency utilizing a phenomenon that a resonance state is established when phases of input current and piston speed are equal to each (Japanese Patent Application Laid-open No. H10-26083).
This conventional method will be explained briefly with reference to a flowchart shown in FIG. 13.
When detection control of resonance frequency is started, in step S20, a sine wave current command value Iref which is input from driving frequency f to the linear compressor is formed. In step S21, current piston speed Vnow is obtained by position information of the piston from a position sensor provided in the linear compressor. In step S22, a position difference between the Iref and Vnow obtained in the above steps, and if the Iref advanced, the procedure is proceeded to step S23, and if the phases are equal to each other, the procedure is proceeded to step S24, and if the Iref delayed, the procedure is proceeded to step S25. In step S23, since the current driving frequency is lower than the resonance frequency and thus, the driving frequency f is increased and the procedure is returned to step S20. In step S24, since the current driving frequency is equal to the resonance frequency, the driving frequency f is not changed and the procedure is returned to step S20. In step S25, since the current driving frequency is higher than the resonance frequency and thus, the driving frequency f is reduced and the procedure is returned to step S20. In this manner, the driving frequency is controlled such that it becomes equal to the resonance frequency using the position information of the piston obtained by the position sensor.
However, in order to employ this method, it is necessary to measure the displacement of the piston in the cylinder. Therefore, a displacement measuring apparatus must be incorporated in the linear compressor. Therefore, there are caused not only a problem that a volume of the linear compressor is increased by a volume of the displacement measuring apparatus, but also a problem that the operational reliability of the displacement measuring apparatus must be secured under rigorous operation conditions such as a temperature, a pressure and refrigerant resistance because the displacement measuring apparatus itself must be enclosed in a shell of the linear compressor.
Further, since it is necessary to differentiate a signal from a displacement sensor and to calculate the position difference between speed and current, a relatively complicated control apparatus such as microcomputer, MPU (micro processor unit) or the like is required.
In view of the above problems, it is an object of the present invention to calculate resonance frequency relatively easily without displacement of a piston in a linear compressor, and to drive the linear compressor efficiently using an inexpensive circuit.
A first aspect of the present invention provides a driving apparatus of a linear compressor for driving a piston in a cylinder by a linear motor to generate compressed gas, comprising an inverter for outputting alternating current which is supplied to the linear motor, a direct current power source for supplying direct current voltage to the inverter, current value commanding means for determining and commanding magnitude of the alternating current, electric power detecting means for detecting input electric power which is supplied to the linear compressor, driving frequency determining means for varying driving frequency of the inverter such that the electric power detected by the electric power detecting means becomes maximum, current waveform commanding means for generating command current waveform from a command current value from the current value commanding means and from a driving frequency determined by the driving frequency determining means, and inverter control means for sending a control signal to the inverter based on the command current waveform from the current waveform commanding means.
According to this aspect, the frequency is varied so that the input electric power which is supplied to the linear motor becomes maximum. That is, to control the effective electric power such that it becomes maximum based on a condition that alternating output current is constant is to control such that a phase of the output current becomes equal to a phase of speed (induction voltage). According to this mode, it is possible to control the linear compressor to resonance frequency without detecting the displacement of the piston.
According to a second aspect of the invention, in the driving apparatus of the linear compressor of the first aspect, the driving apparatus further comprises current detecting means for detecting input current which is supplied to the inverter or output current which is output from said inverter, and voltage detecting means for detecting input voltage of the inverter, and the electric power detecting means calculates input electric power which is supplied to the linear compressor from current detected by the current detecting means and voltage detected by the voltage detecting means, the inverter control means sends a control signal to the inverter such that a deviation between a command current value from the current value commanding means and a detection current value from the current detecting means is reduced.
According to this aspect, direct current and input voltage which are input and output to the inverter are detected, and with a relatively simple calculation in which they are multiplied, it is possible to approximately detect the input electric power which is supplied to the linear motor. The output current value is controlled substantially constantly such that the output current value becomes the command value. That is, to control the effective electric power such that it becomes maximum based on a condition that alternating output current is constant is to control such that a phase of the output current becomes equal to a phase of speed (induction voltage). According to this mode, it is possible to control the linear compressor to resonance frequency without detecting the displacement of the piston.
According to a third aspect of the invention, in the driving apparatus of the linear compressor of the first aspect, the driving apparatus further comprises current detecting means for detecting a smoothened value of sawtooth-like inverter input current as input current or detecting a peak value as output current, and voltage detecting means for detecting input voltage of the inverter, and the electric power detecting means calculates input electric power which is supplied to the linear compressor from current detected by the current detecting means and voltage detected by the voltage detecting means, the inverter control means sends a control signal to the inverter such that a deviation between a command current value from the current value commanding means and a detection current value from the current detecting means is reduced.
According to this aspect, it is possible to detect input current which is supplied to and output current which is output from the inverter by detecting current in only one location using a shunt resistor and a current sensor which are previously provided as a protecting circuit. It is possible to approximately detect the input electric power which is supplied to the linear motor with a relatively simple calculation in which the smoothened value of input current which is supplied to the inverter and the direct current voltage are multiplied. A peak value of the input current corresponding to the output current is substantially constantly controlled such that the peak value becomes the command value and in this state, the frequency is varied such that the electric power becomes maximum. That is, to control the effective electric power such that it becomes maximum based on a condition that the peak value of the input current corresponding to the alternating output current is constant is to control such that a phase of the current becomes equal to a phase of speed (induction voltage). According to this mode, it is possible to control the linear compressor to resonance frequency without detecting the displacement of the piston.
According to a fourth aspect of the invention, in the driving apparatus of the linear compressor of the first aspect, the driving apparatus further comprises current detecting means for detecting input current which is supplied to the direct current power source or output current which is output from the inverter, and voltage detecting means for detecting input voltage which is supplied to the direct current power source, and the electric power detecting means calculates input electric power which is supplied to the linear compressor from current detected by the current detecting means and voltage detected by the voltage detecting means, the inverter control means sends a control signal to the inverter such that a deviation between a command current value from the current value commanding means and a detection current value from the current detecting means is reduced.
According to this aspect, current and voltage of a commercial power source which are input to the direct current power source are detected, and with a relatively simple calculation in which they are multiplied, it is possible to approximately detect the input electric power which is supplied to the linear motor. The output current is substantially constantly controlled such that the output current becomes the command value and in this state, the frequency is varied such that the electric power becomes maximum. That is, to control the electric power such that it becomes maximum based on a condition that alternating output current is constant is to control such that a phase of the current becomes equal to a phase of speed (induction voltage). According to this mode, it is possible to control the linear compressor to resonance frequency without detecting the displacement of the piston.
According to a fifth aspect of the invention, in the driving apparatus of the linear compressor of the first aspect, the driving apparatus further comprises first current detecting means for detecting input current which is supplied to the direct current power source, and second current detecting means for detecting output current which is output from the inverter, and the electric power detecting means calculates input electric power which is supplied to the linear compressor from current detected by the first current detecting means and the direct current power source voltage, the inverter control means sends a control signal to the inverter such that a deviation between a command current value from the current value commanding means and a detection current value from the second current detecting means is reduced.
According to this aspect, the input electric power which is supplied to the linear motor is approximately detected from the current which is input to the direct current power source. That is, the input voltage which is supplied to the direct current power source is stable when the input which is supplied to the direct current power source is a commercial power source. Therefore, the electric power is substantially proportional to the input current, and it is possible to detect the electric power in the easiest manner. The output current is substantially constantly controlled such that the output current becomes the command value and in this state, the frequency is varied such that the electric power becomes maximum. That is, to control the electric power such that it becomes maximum based on a condition that alternating output current is constant is to control such that a phase of the output current becomes equal to a phase of speed (induction voltage). According to this mode, it is possible to control the linear compressor to resonance frequency without detecting the displacement of the piston.
According to a sixth aspect of the invention, in the driving apparatus of the linear compressor of the fifth aspect, the second current detecting means detects a peak value of sawtooth-like inverter input current as inverter output current.
According to this aspect, it is possible to detect alternating output current using a shunt resistor and a current sensor which are previously provided as a protecting circuit. A peak value of the input current corresponding to the output current is substantially constantly controlled such that the peak value becomes the command value and in this state, the frequency is varied such that the electric power becomes maximum. That is, to control the electric power such that it becomes maximum based on a condition that the peak value of the input current corresponding to the alternating output current is constant is to control such that a phase of the current becomes equal to a phase of speed (induction voltage). According to this mode, it is possible to control the linear compressor to resonance frequency without detecting the displacement of the piston.